Active matrix type liquid crystal display and liquid crystal material

ABSTRACT

An active matrix liquid crystal display device is provided, in which an after image remaining after removing an application of a direct current voltage is suppressed. The active matrix liquid crystal display device has a liquid crystal layer containing a liquid crystal molecule having negative dielectric anisotropy and a dopant having a dissociative group.

BACKGROUND OF THE INVENTION

This application is a Divisional application of U.S. Application Ser.No. 10/637,555 filed on Aug. 11, 2003, which is a Continuationapplication of U.S. application Ser. No. 09/866,733 filed on May 30,2001. Priority is claimed based on U.S. application Ser. No. 10/637,555filed on Aug. 11, 2003, which claims priority to U.S. application Ser.No. 09/866,733 filed on May 30, 2001, all of which is incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, andmore particularly, it relates to an active matrix liquid crystal displaydevice of a so-called in-plane switching type.

2. Description of the Related Art

A liquid crystal display realizes display in such a manner that anelectric field is applied to liquid crystal molecules in a liquidcrystal layer sandwiched by a pair of substrates to change theorientation direction of the liquid crystal, so as to cause opticalchange of the liquid crystal layer.

The conventional active matrix liquid crystal display device isrepresented by a twisted nematic (TN) display system, in which thedirection of the application of the electric field to the liquid crystalis set in the direction perpendicular to the substrate plane thatsandwiches the liquid crystal, and the display is realized by utilizingthe optical rotation of the liquid crystal layer.

On the other hand, a liquid crystal display device of a in-planeswitching (IPS) system has been proposed in JP-B-63-21907, U.S. Pat. No.4,345,249, WO 91/10936 and JP-A-6-160878, in which a comb electrode isused, and the direction of the electric field applied to the liquidcrystal is set in the direction parallel to the substrate plane, wherebythe display is realized by utilizing the birefringence of the liquidcrystal.

The in-plane switching system has advantages, such as a wide viewingangle and a low load capacitance, in comparison to the conventional TNsystem, and is being rapidly developed in recent years as a new activematrix liquid crystal display device that superseding the TN system.

In the IPS system, the in-plane switching can be more perfectly realizedin the case where the liquid crystal has negative dielectric anisotropyin comparison to the case of a liquid crystal having positive dielectricanisotropy, as demonstrated in J. of Appl. Phys., vol. 82, No. 4, pp.528-535 (1997) by M. Oh-e, M. Yoneya and K. Kondo. The liquid crystalhaving negative dielectric anisotropy has a dielectric constant in theshort axis direction of the liquid crystal molecule that is larger thanthe dielectric constant in the long axis direction perpendicularthereto, and the liquid crystal having positive dielectric anisotropyhas a dielectric constant in the short axis direction of the liquidcrystal molecule that is smaller than the dielectric constant in thelong axis direction perpindicular thereto.

The perfect realization of the in-plane switching completes enhancementof the viewing angle of the liquid crystal display device includinghalftone. Therefore, the liquid crystal having negative dielectricanisotropy is preferred as a liquid crystal used in the IPS system fromthe foregoing standpoint.

The IPS system employs an opaque metallic comb electrode in a stripeform provided on an inner surface of one of the pair of electrodes.

In recent years, a modified system of the IPS system has been proposedin that the comb electrode is formed with a transparentelectroconductive substance, such as ITO (indium tin oxide), instead ofthe opaque metallic electrode, and is arranged at a shorter intervalthan the conventional IPS system, and a liquid crystal material havingnegative dielectric anisotropy, whereby the entire liquid crystalpresent above the transparent comb electrode can be subjected toorientation change only with an electric field formed at the peripheryof the comb electrode, so as to improve the transmittance and theopening ratio.

Literatures relating to the foregoing proposal include Asia Display1998, pp. 371-374, by S. H. Lee, S. L. Lee and H. Y. Kim and SID digest1999, pp. 202-205, by S. H. Lee, H. Y. Kim and T. Y. Eom.

SUMMARY OF THE INVENTION

The foregoing literatures report that in the IPS system combining theliquid crystal material having negative dielectric anisotropy and theshort interval transparent comb electrode, such transmittance that isclose to the TN system can be realized with maintaining such wideviewing angle characteristics that is equivalent to the IPS system.

It has been known in a liquid crystal display device that in the casewhere a liquid crystal driving voltage waveform having a direct currentvoltage superposed is applied to a liquid crystal layer, the directcurrent voltage (direct current offset voltage) remains in the liquidcrystal layer even when the direct current voltage is removed.

As discussed in S. Matsumoto, Ekishou Display Gijutu (Liquid CrystalDisplay Technique), published by Sangyo Tosho Co., Ltd., Chap. 2, pp.70-73, the application of the driving voltage waveform having a directcurrent voltage superposed to the liquid crystal layer may occur in anactive matrix liquid crystal display device in an ordinary liquidcrystal operation due to the structure of the active driving element ofthe liquid crystal display device, and it is difficult to completelyprevent the superposing phenomenon of a direct current voltage whengradation display is conducted. The phenomenon is common to both the TNsystem and the IPS system conventionally employed.

The remaining direct current voltage affects the brightness in liquidcrystal display devices of both the TN system and the IPS system, anddifference in brightness is caused between a part applied with thedirect current voltage and a part not applied therewith or between partshaving different intensities of the applied direct current voltage.

Therefore, in the case, for example, where texts or graphics aredisplayed under ordinary driving conditions for a long period of time,such a phenomenon occurs that the texts or graphics that have beendisplayed are displayed for a certain period after turning off thedisplay.

As a result, uniformity of display is impaired. Such a phenomenon iscalled as an after image of a liquid crystal display device, which isgradually decreased in intensity with the lapse of time after formationthereof and is finally disappeared, but there are cases where a periodof 30 minutes or more is required to disappear upon viewing with thenaked eye.

As a mechanism that when a direct current voltage is applied, the directcurrent offset voltage remains in a liquid crystal layer, a modelexplaining by behavior of ions in the liquid crystal layer in theconventional TN system as an example has been proposed in Shingaku Gihou(Technical Research Report of Institute of Electronics, Information andCommunication Engineers), EID96-89, pp. 29-34 (1997-01).

According to the model, a direct current voltage charged in an orientedfilm and absorption of ions on an orientation film for directing theliquid crystal are considered as factors of the direct current voltageremaining in the liquid crystal layer, and it sums up that the remanenceof the direct current voltage for several minutes is caused by chargingand relaxation of the orientation film, and the remanence of the directcurrent voltage for a longer period is caused by absorption of ions onthe orientation film.

The IPS system suffers more frequent occurrence of the after image thanthe TN system. In the TN system, only liquid crystal orientationcontrolling layers and a liquid crystal layer are present between apixel electrode and a counter electrode, and an electric field isapplied to the pixel electrode, the liquid crystal orientationcontrolling layer, the liquid crystal layer, the liquid crystalorientation controlling layer and the counter electrode in this order.

On the other hand, the IPS system has an insulating layer in addition tothe liquid crystal layer and the liquid crystal orientation controllinglayers between the pixel electrode and the counter electrode, and theelectric field is applied to the pixel electrode, the liquid crystalorientation controlling layer, the liquid crystal layer, the liquidcrystal orientation controlling layer, the insulating layer and thecounter electrode in this order.

That is, because charging and relaxation of the orientation films andthe insulating film are considered while only the orientation film isconsidered for the remanence of the direct current voltage in the TNsystem, the after image is liable to occur in the IPS system as comparedto the TN system.

In TN system, the after image is liable to occur when an insulatinglayer is arranged on the pixel electrode or the counter electrode tosandwich the insulating layer between pixel electrode and the counterelectrode, to which the electric field is applied.

However, the occurrence of the after image can be suppressed by openingholes on the insulating film at positions above the pixel electrode andthe counter electrode, so as to apply the electric field on the pixelelectrode, the liquid crystal orientation controlling layer, the liquidcrystal layer, the liquid crystal orientation controlling layer and thecounter electrode in this order.

JP-A-7-159786 proposes a method for suppressing the remanence phenomenonof the direct current voltage caused by charging and relaxation of theorientation film by optimizing the dielectric constant and the specificresistance of the orientation film and the liquid crystal. In order tosuppress the after image by accelerating the charging and relaxation ofthe orientation film and the insulating film, it is effective that theliquid crystal has a lower specific resistance.

The specific resistance of the liquid crystal can be decreased by addinga substance that decreases the specific resistance of the liquidcrystal. For example, JP-A-11-302652 proposes that the specificresistance of a liquid crystal can be adjusted by adding an oxidativecompound to the liquid crystal.

The after image causes no problem when a liquid crystal containing theoxidative compound is used in the IPS system using a liquid crystalmaterial having positive dielectric anisotropy and the IPS systemcombining a liquid crystal material having positive dielectricanisotropy and a short interval transparent comb electrode.

However, the occurrence of the after image cannot be completely avoidedby using the oxidative compound in the IPS system using a liquid crystalmaterial having negative dielectric anisotropy and the IPS systemcombining a liquid crystal material having negative dielectricanisotropy and a short interval transparent comb electrode.

The oxidative compound has a molecular structure that is similar to theliquid crystal material having positive dielectric anisotropy. That is,one of the both ends in the long axis of the molecule is formed with agroup having polarity other than a group having no polarity or extremelyweak polarity, such as an alkyl group or an alkoxy group.

The other of the ends is formed with a group having high polarity, suchas a cyano group or a fluorine-containing group, and it is polarized inthe longer axis of the molecule rather than the shorter axis of themolecule.

The liquid crystal molecule having positive dielectric anisotropy isalso polarized in the longer axis of the molecule rather than theshorter axis of the molecule. In other words, the liquid crystalmaterial having positive dielectric anisotropy and the oxidativecompound agree to each other in the molecular axis direction and thepolarizing direction. It is therefore considered that the remainingdirect current voltage can be effectively relaxed.

However, in the case of the liquid crystal material having negativedielectric anisotropy, the both ends in the longer axis direction of themolecule are formed with a group having no polarity or extremely weakpolarity, such as an alkyl group or an alkoxy group, and one end in theshorter axis of the molecule is formed with a group having highpolarity, such as a cyano group and a fluorine-containing group.Therefore, it is polarized in the shorter axis of the molecule ratherthan the longer axis of the molecule.

As described in the foregoing, the liquid crystal material havingnegative dielectric anisotropy does not agree to the oxidative compound,which has a molecular structure that is similar to the liquid crystalmaterial having positive dielectric anisotropy, in the molecular axisdirection and the polarizing direction. Therefore, it is considered thatthe remaining direct current voltage cannot be effectively relaxed.

The invention has been developed to solve the foregoing problemsassociated with the conventional art.

An object of the invention is to provide an active matrix liquid crystaldisplay device of an IPS system that is difficult to cause a state ofununiform display remaining after application of a direct currentvoltage, i.e., an after image, in an IPS system using a liquid crystalmaterial having negative dielectric anisotropy and an IPS systemcombining a liquid crystal material having negative dielectricanisotropy and a short interval transparent comb electrode.

Another object of the invention is to provide an active matrix liquidcrystal display device of an IPS system that is difficult to cause anafter image by adding a dissociative dopant and modifying the shape ofthe electrode even in the case where a liquid crystal material havingpositive dielectric anisotropy.

In order to accomplish the objects, the invention relates to an activematrix liquid crystal display device comprising

a pair of substrates;

a liquid crystal layer sandwiched by said pair of substrates;

orientation films defining an orientation direction of a liquid crystalmolecule of said liquid crystal layer, said orientation films beingarranged between said pair of substrates and said liquid crystal layer;and

a pixel electrode and a counter electrode applying a voltage to saidliquid crystal layer,

said liquid crystal molecule of said liquid crystal layer havingnegative dielectric anisotropy, and said liquid crystal layer containinga dopant having a dissociative group.

A liquid crystal display device causing less after image can beprovided.

In order to accomplish the objects, the invention relates to an activematrix liquid crystal display device comprising a pair of substrates, atleast one of which is transparent; liquid crystal orientationcontrolling layers formed on surfaces of the pair of substrates facingeach other; a liquid crystal layer comprising a liquid crystalcomposition having negative dielectric anisotropy arranged between thepair of substrate to make in contact with the liquid crystal orientationcontrolling layers (orientation films); a pixel electrode and a counterelectrode formed on one of the pair of substrates through an insulatingfilm; and an active element connected to the pixel electrode and thecounter electrode, the liquid crystal layer containing a dopant having adissociative group only in a shorter axis direction of the molecule andhaving an alkyl group or an alkoxy group on both ends of a molecularaxis direction.

According to the liquid crystal display device, the liquid crystalmaterial having negative dielectric anisotropy and the dopant having adissociative group only in a shorter axis direction of the moleculeagree to each other in the molecular axis direction and the polarizingdirection.

Therefore, the remaining direct current voltage can be effectivelyrelaxed, and a liquid crystal display device causing less after imagecan be provided.

The dopant having a dissociative group referred herein means an acidicdissociative substance or a basic dissociative substance, or in otherwords, a substance generating an H⁺ ion through dissociation by itselfin a polar solvent or generating an OH⁻ ion through a reaction withwater.

Specific examples thereof include a carboxylic acid (including ananhydride thereof), an amide, an amine and an alcohol. When thesesubstances are added to a liquid crystal, the ion concentration in theliquid crystal is increased, so as to decrease the specific resistance.

It is preferred that the pixel electrode and the counter electrode aretransparent electrode formed with a transparent electrode, such as ITO,and electric insulation between the pixel electrode and the counterelectrode is maintained by a transparent insulating film. For example,the pixel electrode may be a short interval transparent comb electrode,and the counter electrode may be a solid electrode. The transparentinsulating film may be constituted, for example, with IZO, siliconnitride, titanium oxide, silicon oxide and a mixture thereof.

When the dopant has the following structure represented by the generalformula (I), it can effectively relax the remaining direct currentvoltage, so as to provide a liquid crystal display device exhibitingless after image. The dopant having the following structure has amolecular structure that is similar to a liquid crystal material havingnegative dielectric anisotropy.

That is, both ends in the longer axis direction of the molecule areformed with a group having polarity other than a group having nopolarity or extremely weak polarity, such as an alkyl group or an alkoxygroup.

Since it has a dissociative group in the shorter axis direction of themolecule, it is strongly polarized in the shorter axis direction of themolecule.

The liquid crystal molecule having negative dielectric anisotropy isalso polarized in the shorter axis direction of the molecule rather thanthe longer axis direction of the molecule.

The liquid crystal material having negative dielectric anisotropy andthe dopant having the following structure agree to each other in themolecular axis and the polarizing direction.

Accordingly, the remaining direct current voltage can be effectivelyrelaxed. 1

wherein Y₁ represents any one of —COOH, —CONH₂, —NH₂, —OH, —NHR or —NR₂;Y₂ represents anyone of hydrogen, —F, —CN, —COOH, —CONH, —NH₂ or —OH; Y₃represents any one of hydrogen, —F, —CN, —COOH, —CONH, —NH₂ or —OH; Y₄represents any one of hydrogen, —F, —CN, —COOH, —CONH, —NH₂ or —OH; X₁represents any one of a single bond, —CO—O—, —O—CO—, —COCH₂—, —CH₂—CO—,—CH₂O—, —OCH₂—, —CH₂—CH₂— or —CH═CH—; X₂ represents any one of a singlebond, —CO—O—, —O—CO—, —COCH₂—, —CH₂—CO—, —CH₂O—, —OCH₂—, —CH₂—CH₂— or—CH═CH—; A₁ represents any one of a single bond, a phenylene group or acyclohexylene group; A₂ represents any one of a single bond, a phenylenegroup or a cyclohexylene group; R₁ represents any one of an alkyl groupor an alkoxy group; and R₂ represents any one of an alkyl group or analkoxy group.

Furthermore, when the dopant has the following structure represented bythe general formula (II), it can more effectively relax the remainingdirect current voltage, so as to provide a liquid crystal display deviceexhibiting less after image. The dopant having the following structurehas a molecular structure that is further similar to a liquid crystalmaterial having negative dielectric anisotropy. That is, both ends inthe longer axis direction of the molecule are formed with a group havingpolarity other than a group having no polarity or extremely weakpolarity, such as an alkyl group or an alkoxy group.

In this application, single bond means direct connection. In case X1 issingle bond, for example, A1 connect directly to benzene structure.

Since it has a highly dissociative group or a group having high polarityat one end of the shorter axis direction of the molecule, it is stronglypolarized in the shorter axis direction of the molecule. Since theliquid crystal material having negative dielectric anisotropy has agroup having high polarity, such as a cyano group or afluorine-containing group, at one end of the shorter axis direction ofthe molecule, it is polarized in the shorter axis direction of themolecule rather than the longer axis direction of the molecule.

The liquid crystal material having negative dielectric anisotropy andthe dopant having the following structure agree to each other in themolecular axis and the polarizing direction. Accordingly, the remainingdirect current voltage can be effectively relaxed. 2

wherein Y₁ represents any one of —COOH, —CONH₂, —NH₂, —OH, —NHR or —NR₂;Y₂ represents any one of hydrogen, —F, —CN, —COOH, —CONH, —NH₂ or —OH;X₁ represents any one of a single bond, —CO—O—, —O—CO—, —COCH₂—,—CH₂—CO—, —CH₂O—, —OCH₂—, —CH₂—CH₂— or —CH═CH—; X₂ represents any one ofa single bond, —CO—O—, —O—CO—, —COCH₂—, —CH₂—CO—, —CH₂O—, —OCH₂—,—CH₂—CH₂— or —CH═CH—; A₁ represents any one of a single bond, aphenylene group or a cyclohexylene group; A₂ represents any one of asingle bond, a phenylene group or a cyclohexylene group; R₁ representsany one of an alkyl group or an alkoxy group; and R₂ represents any oneof an alkyl group or an alkoxy group.

The content of the dopant in the liquid crystal is generally 100 ppm(1×10⁻⁴% by weight) or more, and preferably 1,000 ppm or more. When anon-liquid crystal substance is incorporated in a liquid crystal, thecharacteristics of the liquid crystal (liquid crystal property) isdeteriorated, and when a too large amount of the non-liquid crystalsubstance is incorporated, such a temperature range in that the liquidcrystal behaves as a liquid crystal nature becomes narrow. In theinvention, since the dopant is incorporated in the liquid crystal in anamount of 100 ppm or more, preferably 1,000 ppm or more, the after imagecan be suppressed while decrease of the liquid crystal property of theliquid crystal is suppressed to the allowable range to constitute anactive matrix liquid crystal display device, whereby a liquid crystal.display device having excellent liquid crystal characteristics and lessafter image can be provided.

When the specific resistance of the liquid crystal is from 1.0×10⁹ to1.0×10¹² Ω·cm, a liquid crystal display device having less after imagecan be provided. When the liquid crystal has a specific resistance ofmore than 1.0×10¹² Ω·cm, the effect of suppressing the after imagecannot be conspicuously obtained, and when the liquid crystal has aspecific resistance of less than 1.0×10⁹ Ω·cm, high display qualitycannot be maintained.

The orientation film as the liquid crystal orientation controlling layeris formed to have a film thickness of from 20 nm to 300 nm. When thefilm thickness of the orientation film is less than 20 nm, theuniformity of the orientation film is deteriorated since the unevennessof the surface of the ITO film or the IZO film, which is formed underthe orientation film, is from 10 nm to 20 nm, so as to cause displayunevenness and to cause printing unevenness of the orientation film uponforming the orientation film. When the film thickness of the orientationfilm is more than 300 nm, the orientation film is ununiformly dried,which causes display unevenness.

The insulating film is formed to have a film thickness of from 0.1 μm to4 μ. When the film thickness of the insulating film is less than 0.1 μ,the insulating property of the film is deteriorated, and when it exceeds4 μ, the after image becomes conspicuous.

As the liquid crystal having negative dielectric anisotropy, a liquidcrystal containing a liquid crystal molecule having a difluorinatedbenzene structure in the molecule and a liquid crystal containing aliquid crystal molecule having a dicyanobenzene structure in themolecule can be used.

Furthermore, a liquid crystal containing both a liquid crystal moleculehaving a difluorinated benzene structure in the molecule and a liquidcrystal molecule having a dicyanobenzene structure in the molecule canalso be used. A liquid crystal containing a liquid crystal moleculehaving a monocyanocyclohexane structure in the molecule can also be usedas the liquid crystal having negative dielectric anisotropy.

A liquid crystal containing both a liquid crystal molecule having adifluorinated benzene structure in the molecule and a liquid crystalmolecule having a monocyanocyclohexane structure in the molecule canalso be used.

Moreover, in the case of a liquid crystal having positive dielectricanisotropy is used, the dissociative dopant can be added, and thestructures of the pixel electrode and the counter electrode arenormalized, whereby the occurrence of the after image can be suppressed.

One means is an active matrix liquid crystal display device comprising apair of substrates; a liquid crystal layer sandwiched by said pair ofsubstrates; orientation films defining an orientation direction of aliquid crystal molecule of said liquid crystal layer, said orientationfilms being arranged between said pair of substrates and said liquidcrystal layer; and a pixel electrode and a counter electrode applying avoltage to said liquid crystal layer, said liquid crystal molecule ofsaid liquid crystal layer having positive dielectric anisotropy, andsaid liquid crystal layer containing a dopant having a dissociativegroup.

A means with a liquid crystal composition comprising from 100 ppm to1,000 ppm of a dopant having a dissociative group only in a shorter axisdirection of a molecule and having an alkyl group or an alkoxy group onboth ends of said shorter axis direction of a molecule is effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plane view showing a constitutional example of one pixel anda periphery thereof of a liquid crystal display part of an active matrixcolor liquid display device.

FIG. 2 is a cross sectional view on line II-II in FIG. 1.

FIG. 3 is a cross sectional view of a thin film transistor (TFT) on lineIII-III in FIG. 1.

FIG. 4 is a cross sectional view of a storage capacitance (Cstg) formingpart on line IV-IV in FIG. 1.

FIG. 5 is a diagram showing the relationship of an electric fieldapplication direction and a rubbing direction to a transmitting axis ofa polarizing plate.

FIG. 6 is a plane view showing a structure of a matrix periphery of adisplay panel.

FIG. 7A is a diagram showing a panel edge part having a gate signalterminal, and FIG. 7B is a diagram showing a panel edge part having noterminal for external connection.

FIGS. 8A and 8B are a plane view and a cross sectional view,respectively, of an example of a structure in the vicinity of aconnecting part of a gate terminal GTM and a gate line GL.

FIGS. 9A and 9B are a plane view and a cross sectional view,respectively, of an example of a structure in the vicinity of aconnecting part of a drain terminal DTM and a drain signal line DL.

FIGS. 10A and 10B are a plane view and a cross sectional view,respectively, of an example of a structure in the vicinity of aconnecting part of a counter electrode terminal CTM and a common basline CB with a common voltage signal line CL.

FIG. 11 is a diagram showing a circuit diagram of a matrix part and aperiphery thereof of an active matrix color liquid crystal displaydevice.

FIG. 12 is a diagram showing a driving waveform of an active matrixcolor display device according to the invention.

FIG. 13 is a top view showing a liquid crystal display panel having aperipheral driving circuit mounted thereon.

FIGS. 14A to 14C are diagrams showing a production process of asubstrate SUB1.

FIGS. 15D to 15F are diagrams showing a production process of asubstrate SUB1 subsequent FIGS. 14A to 14C.

FIG. 16 is a plane view showing one pixel of another example of a liquidcrystal display part of an active matrix color liquid crystal displaydevice.

FIGS. 17A and 17B are plane views showing one pixel of further examplesof a liquid crystal display part of an active matrix color liquidcrystal display device.

FIGS. 18A and 18B are plane views showing one pixel of further examplesof a liquid crystal display part of an active matrix color liquidcrystal display device.

FIGS. 19A and 19B are plane views showing one pixel of further examplesof a liquid crystal display part of an active matrix color liquidcrystal display device.

FIG. 20 is a cross sectional view showing one pixel of a further exampleof a liquid crystal display part of an active matrix color liquidcrystal display device.

FIG. 21 is a cross sectional view showing one pixel of a further exampleof a liquid crystal display part of an active matrix color liquidcrystal display device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described below with reference tothe drawings. An example of an active matrix color liquid crystaldisplay device, to which the invention is applied, will be described. Inthe drawings, the same symbols are attached to the parts having the samefunctions to omit the repetition of explanation.

CL EXAMPLE 1

Plane Structure of Matrix Part (Pixel Part)

FIG. 1 is a plane view showing one pixel and a periphery thereof of anactive matrix color liquid display device according to the invention.The following description refers to a so-called thin film transistorliquid crystal display device using a thin film transistor (TFT) as anactive matrix element.

As shown in FIG. 1, the pixel is arranged in the region surrounded by agate signal line (a scanning signal line or a horizontal signal line)GL, a common voltage signal line (a counter electrode line) CL andadjacent two drain signal lines (image signal lines or vertical signallines) DL crossing each other.

These signal lines each is formed with an opaque metallic electrode. Thegate signal line GL and the common voltage signal line CL are liedhorizontally in FIG. 1, and pluralities thereof are arranged vertically.The image signal line DL is laid vertically, and a plurality thereof isarranged horizontally.

The pixel electrode PX is formed with an ITO transparentelectroconductive film and is electrically connected with (a sourceelectrode SD1 of) the thin film transistor TFT via a through hole. Thecounter electrode CT is also formed with ITO and is electricallyconnected with the common voltage signal line CL. The symbol SD2 denotesa drain electrode and AS denotes a semiconductor layer.

The pixel electrode PX is formed to has a comb form, each of which is along electrode extending in the vertical direction in FIG. 1. Thecounter electrode CT is a solid transparent electrode, and the opticalstate of a liquid crystal composition LC is controlled by an electricfield generated between the pixel electrode PX and the counter electrodeCT.

The gate signal line GL is to transmit the gate signal to the thin filmtransistor TFT of the respective pixels, and the drain signal line DL isto supply the drain signal voltage to the pixel electrode PX of therespective pixels via (a drain electrode SD2 of) the thin filmtransistor TFT. The common voltage signal line CL is to supply thecommon voltage to the counter electrode CT of the respective pixels.

The common voltage signal line CL formed with a metallic electrode isformed to surround the sides of the drain signal line DL, and it alsofunctions as a light shielding layer for preventing unnecessary lightleakage on the sides of the drain line caused by influence of anelectric field formed by the potential of the drain electrode.

The electrode width W and the electrode interval L of the pixelelectrode PX of a comb form are changed depending on the liquid crystalmaterial used. This is because since the intensity of the electric fieldthat attains the maximum transmissibility varies depending on the liquidcrystal material, the electrode interval is set depending on the liquidcrystal material, so as to obtain the maximum transmissibility withinthe range of the maximum amplitude of the signal voltage determined bythe withstanding voltage of the drain signal driving circuit (signaldriver) used.

The width of the pixel electrode is set at a range of from 1 μ to 15 μ,and in this example, it is set at 4 μ taking the opening ratio and theproductivity of the electrode into consideration. The electrode intervalL is set at a range of from 1 μ to 10 μ, and in this example, it is setat 4 μ for realizing the driving voltage of 10 V or less.

Cross Sectional Structure of Matrix Part (Pixel Part)

FIG. 2 is a cross sectional view on line II-II in FIG. 1, FIG. 3 is across sectional view of the thin film transistor TFT on line III-III inFIG. 1, and FIG. 4 is a cross sectional view of a storage capacitance(Cstg) forming part on line IV-IV in FIG. 1.

As shown in FIGS. 2 to 4, on a lower transparent glass substrate SUB1arranged under the liquid crystal composition layer (hereinaftersometimes simply referred to as a liquid crystal) LC, the thin filmtransistor TFT, the storage capacitance Cstg and the electrode group areformed, and on a upper transparent glass electrode SUB2, a color filterFIL and a light shielding black matrix pattern BM are formed.

Orientation films ORI1 and ORI2, which are liquid crystal orientationcontrolling layers for controlling the initial orientation of the liquidcrystal, are formed on the inner surfaces (on the side of the liquidcrystal LC) of the transparent glass substrates SUB1 and SUB2.Polarizing plates POL1 and POL2 are provided on the outer surfaces ofthe transparent glass substrates SUB1 and SUB2.

This example has such a structure as shown in FIGS. 2 to 4 that thecounter electrode CT as solid ITO and the gate signal line GL are in thesame layer, and the pixel electrode PX as comb form ITO is formed on aprotective insulating film PSV formed on the signal line DL.

Therefore, in the cross sectional view, the pixel electrode PX and thecounter electrode CT are sandwiched by a gate insulating film GI and theprotective insulating film PSV, which form the storage capacitance Cstg.

The common signal line CL is in contact with the counter electrode CT inthe same layer. The gate insulating film GI and the protectiveinsulating film PSV may be formed with SiO₂ or Si_(x)N_(y).

In addition to the structure of the pixel electrode and the counterelectrode shown in FIG. 2, the following structures may be employed. Asshown in FIG. 20, a pixel electrode PX is formed with solid ITO, and acounter electrode CT having a comb electrode part is arranged above thepixel electrode PX through a protective insulating film PSV2. As shownin FIG. 21, a pixel electrode is formed with solid ITO, and a counterelectrode CT having a comb electrode part is arranged above the pixelelectrode PX through a protective insulating film PSV1.

TFT Substrate

The structure of the lower transparent glass substrate SUB1 (a TFTsubstrate) will be described in detail below.

Thin Film Transistor TFT

FIG. 3 shows a cross sectional view of the part of the thin filmtransistor. The thin film transistor TFT function in such a manner thatwhen a positive bias is applied to a gate electrode GT, the channelresistance between the source and the drain is decreased, and when thebias is zero, the channel resistance is increased.

The thin film transistor TFT has a gate electrode GT, a gate insulatingfilm GI, an i type (intrinsic, i.e., doped with no impurity determiningthe conductive type) semiconductor layer AS comprising i type amorphoussilicon (Si), and a pair of electrodes (a source electrode SD1 and adrain electrode SD2).

The source electrode SD1 and the drain electrode SD2 are fundamentallydetermined by the bias polarity between them, and the polarity isrepeatedly inverted during operation of the circuit of the liquidcrystal display device. Therefore, it is understood that the sourceelectrode SD1 and the drain electrode SD2 are interchanged by each otherduring operation. However, in the following description, one of them isreferred to as a source electrode and the other is referred to as adrain electrode for convenience.

Gate Electrode GT

The gate electrode GT is formed continuously from the gate signal lineGL, and a part of the region of the gate signal line GL is formed as thegate electrode GT. The gate electrode GT is a region beyond the activeregion of the thin film transistor TFT.

In this example, the gate electrode GT is formed with a single layerelectroconductive film g1. The electroconductive film g1 may be, forexample, a chromium-molybdenum (Cr—Mo) alloy film formed by sputtering,but it is not limited thereto. The electroconductive film may have atwo-layer structure of different metals.

Gate Signal Line GL

The gate signal line GL is formed with a single layer electroconductivefilm g1. The electroconductive film g1 of the gate signal line GL isformed in the same production process as the electroconductive film g1of the gate electrode GT and is integrated therewith.

A gate voltage GV is supplied from an outer circuit to the gateelectrode GT via the gate signal line GL. In this example, achromium-molybdenum (Cr—Mo) alloy film formed by sputtering, forexample, is used as the electroconductive film g1.

The material of the gate signal line GL and the gate electrode GT is notlimited to a chromium-molybdenum alloy and may be, for example, atwo-layer structure comprising aluminum or an aluminum alloy wrappedwith a chromium-molybdenum alloy for decreasing the resistance.

Common Voltage Signal Line CL

The common voltage signal line CL is formed with an electroconductivefilm g1. The electroconductive film g1 of the common voltage signal lineCL is formed in the same production process as the electroconductivefilm of the gate signal line GL and the gate electrode GT and isintegrated with the counter electrode CT.

A common voltage Vcom is supplied from an outer circuit to the counterelectrode CT via the common voltage signal line CL.

The material of the common voltage signal line CL is not limited to achromium-molybdenum alloy and may be, for example, a two-layer structurecomprising aluminum or an aluminum alloy wrapped with achromium-molybdenum alloy for decreasing the resistance.

Insulating Film GI

The insulating film GI is used as a gate insulating film for applying anelectric field to the semiconductor layer AS associated with the gateelectrode GT in the thin film transistor TFT. The insulating film GI isformed as an upper layer of the gate electrode GT and the gate signalline GL.

As the insulating film GI, for example, a silicon nitride film formed byplasma CVD is selected, which is formed to have a thickness of from 100nm to 4 μ (about 350 nm in this example).

The gate insulating film GI also functions as an interlevel dielectricfilm of the gate signal line GL and the common voltage signal line CLwith the drain signal line DL, and contributes to electric insulationthereof.

i Type Semiconductor Layer AS

The i type semiconductor layer AS formed with an amorphous siliconsemiconductor at a thickness of from 15 nm to 250 nm (about 120 nm inthis example). A layer d0 is an N(+) type amorphous siliconsemiconductor layer doped with phosphorous (P) for ohmic contact, whichis left at a part, where the i type semiconductor layer AS is presentunder the layer d0, and an electroconductive layer d1 is present abovethe layer d0.

The i type semiconductor layer AS and the layer d0 are also provided ata crossing part (a crossover part) of the gate signal line GL and thecommon voltage signal line CL with the drain signal line DL. The i typesemiconductive layer AS at the crossing part decreases the short circuitof the gate signal line GL and the common voltage signal line CL withthe drain signal line DL.

Source Electrode SD1 and Drain Electrode SD2

The source electrode SD1 and the drain electrode SD2 each is formed withthe electroconductive layer d1 in contact with the N(+) typesemiconductive layer d0. A Cr—Mo film has low stress and thus can beformed to have a relatively large film thickness to contribute todecrease of the resistance of the line. The Cr—Mo film has good adhesionproperty to the N(+) type semiconductor layer d0.

Drain Signal Line DL

The drain signal line DL is formed in the same layer as the sourceelectrode SD1 and the drain electrode SD2. The drain signal line DL isformed as integrated with the drain electrode SD2.

In this example, the electroconductive film d1 is a chromium-molybdenumalloy (Cr—Mo) film formed by sputtering at a thickness of from 50 nm to300 nm (about 250 nm in this example). The Cr—Mo film has low stress andthus can be formed to have a relatively large film thickness tocontribute to decrease of the resistance of the line.

The Cr—Mo film has good adhesion property to the N(+) type semiconductorlayer d0. As the electroconductive film d1, other than the Cr—Mo film, ahigh melting point metal (such as Mo, Ti, Ta and W) film and a highmelting point metal silicide (such as MoSi₂, TiSi₂, TaSi₂ and WSi₂) filmmay also be used, and an accumulated layer structure with aluminum mayalso be used.

Storage Capacitance Cstg

An electroconductive film ITO2 forming the storage capacitance Cstg isformed to overlap an electroconductive film ITO1 forming the counterelectrode CT. The overlap constitutes a storage capacitance (electricalcapacitance element) Cstg between the pixel electrode PX and the counterelectrode CT as understood from FIG. 2.

The dielectric film of the storage capacitance Cstg is formed with theprotective film PSV and the insulating film GI used as the gateinsulating film of the thin film transistor TFT. As shown in FIG. 4,from a plane view, the storage capacitance Cstg is formed as anoverlapping part of the pixel electrode PX and the counter electrode CTinside the pixel.

Protective Film PSV

The protective film PSV is formed on the thin film transistor TFT. Theprotective film PSV is provided mainly for protecting the thin filmtransistor from moisture, and a film having high transparency and highmoisture resistance is used.

The protective film PSV is formed, for example, with a silicon oxidefilm or a silicon nitride film formed by using a plasma CVD apparatus ata film thickness of from 0.1 μ to 1 μ. The protective film PSV isremoved to expose outer connecting terminals DTM and GTM.

With respect to the thickness of the protective film PSV and theinsulating film GI, the thickness of the protective film PSV is madelarge taking the protection effect into consideration, and the thicknessof the insulating film GI is made thin taking the mutual conductance gmof the transistor into consideration. The protective film PSV may havean accumulated structure of an organic film, such as polyimide, having arelatively large thickness of from 2 μ to 3 μ.

Pixel Electrode PX

The pixel electrode PX is formed with ITO as a transparentelectroconductive material, and forms a storage capacitance with thecounter electrode CT, which is similarly formed with ITO. In thisexample, explanation is made by using ITO as the transparentelectroconductive material, but the same effect can be obtained by usingindium-zinc oxide (IZO).

Counter Electrode CT

The counter electrode CT is formed with ITO and is connected to thecommon voltage signal line CL in the same layer. It is constituted insuch a manner that the common voltage Vcom is applied to the counterelectrode CT.

In this example, the common voltage Vcom is set at a potential that islower than the intermediate direct current potential between the minimumlevel driving voltage Vdmin and the maximum level driving voltage Vdmaxapplied to the drain signal line DL by the feed through voltage ΔVsformed upon turning off the thin film transistor TFT.

In this example, explanation is made by using ITO as the transparentelectroconductive material, but the same effect can be obtained by usingIZO.

Color Filter Substrate

The upper transparent glass substrate SUB2 (color filter substrate) willbe described in detail with reference to FIGS. 1 and 2.

Light Shielding Film BM

On the upper transparent glass substrate SUB2, as a BM boundary lineshown by a heavy line in FIG. 1, a light shielding film BM (a so-calledblack matrix) is formed to prevent decrease of contrast caused byemission of transmitted light from an unnecessary gap (a gap other thanthat between the pixel electrode PX and the counter electrode CT) to thedisplay surface.

The light shielding film BM also functions to prevent emission of outerlight or back light incident on the i type semiconductor layer AS. Thatis, the i type semiconductor layer As of the thin film-transistor TFT isvertically sandwiched by the light shielding film BM and the relativelylarge gate electrode GT (FIG. 3) so as to prevent from irradiation withouter natural light and back light.

Although the light shielding film BM in FIG. 1 is shown for only onepixel, it is formed to have openings inside the respective pixels. Thepattern thereof used herein is a mere example.

At a part where the direction of the electric field is disturbed, suchas an edge of the comb electrode, the display has one-to-onecorrespondence to the image information inside the pixel, and becomesblack in case of black or white in case of white. Therefore, it can beused as a part of display.

However, the light shielding film BM necessarily has a shieldingfunction to light. In particular, at the gap between the pixel electrodePX and the counter electrode CT, the optical density thereof isnecessarily 3 or more for preventing cross talk in the drain signal linedirection (vertical smear).

While the light shielding film BM may be formed with a metal havingelectroconductivity, such as Cr, it is preferably formed with a filmhaving high insulation property to prevent influence on the electricfield between the pixel electrode PX and the counter electrode CT.

In this example, a black organic pigment is mixed with a resist materialand formed to a thickness of about 1.2 μ. In order to improve theshielding property to light, carbon and titanium oxide (Ti_(x)O_(y)) maybe mixed in an amount of such a range that can maintain the insulatingproperty of 10⁸ Ω·cm or more, which does not affect the electric fieldinside the liquid crystal composition layer.

Since the light shielding film BM comparts the effective display regionsof the respective lines, it also has a function of clarifying thecontour of the pixel of the respective lines. The light shielding filmBM is also formed in the form of frame on the periphery, the pattern ofwhich is formed continuously from the pattern of the matrix part shownin FIG. 1.

The light shielding film BM in the periphery is extended beyond a sealpart SL (see FIG. 7), so as to prevent invasion of leaked light, such asreflected light, caused by practical implementation, such as a personalcomputer, to the matrix part, and also to prevent leakage of light, suchas back light, to the outside of the display area.

The light shielding film BM is terminated inside the edge of thesubstrate SUB2 by about 0.3 mm to 1.0 mm, so as to form around the cutregion of the substrate SUB2.

Color Filter FIL

The color filter FIL is formed in a stripe form comprising repeatingcolors, red, green and blue, at the counter position of the pixels. Thecolor filter FIL is formed to overlap the light shielding film BM.

The color filter FIL can be formed in the following manner. A dyeingbase material, such as an acrylic resin, is formed on the surface of theupper transparent glass substrate SUB2, and the dyeing base material onthe region other than the red filter forming region is removed by thephotolithography technique.

Thereafter, the remaining dyeing base material is dyed with a redpigment, followed by subjecting to a fixing treatment, so as to form ared filter R. A green filter G and a blue filter B are then formed inthe same manner. The dyeing can also be conducted with a dye.

Overcoating Film OC

An overcoating film OC is provided for preventing leakage of the dyes ofthe color filter FIL to the liquid crystal composition layer LC and forflattening steps formed by the color filter FIL and the light shieldingfilm BM.

The overcoating layer is formed, for example, with a transparent resinmaterial, such as an acrylic resin and an epoxy resin. An organic film,such as polyimide having good flowability, can also be used as theovercoating layer.

Liquid Crystal Layer and Polarizing Plate

The liquid crystal layer, the orientation film and the polarizing platewill be described below.

Liquid Crystal Layer

In this example, a nematic liquid crystal having a negative dielectricanisotropy At of a value of 4.0 and a refractive index anisotropy Δn of0.100 (589 nm, 20° C.) containing a liquid crystal molecule having adifluorinated benzene structure in the molecule is used as the liquidcrystal.

In addition to the above, a liquid crystal having a dielectricanisotropy Δξ, a liquid crystal containing a liquid crystal moleculehaving a dicyanobenzene structure in the molecule, a liquid crystalcontaining a liquid crystal molecule having a difluorinated benzenestructure in the molecule, a liquid crystal containing a liquid crystalmolecule having a dicyanobenzene structure in the molecule, a liquidcrystal containing a liquid crystal molecule having amonocyanocyclohexane structure in the molecule and a liquid crystalcontaining both a liquid crystal molecule having a difluorinated benzenestructure in the molecule and a liquid crystal molecule having amonocyanocyclohexane structure in the molecule can be used. The liquidcrystal is not limited to the foregoing composition and may be used asfar as it is a liquid crystal having negative dielectric anisotropy.

The thickness of the liquid crystal composition layer (gap) is 3.0 μ,and the retardation is 0.30 μ. It is combined with the orientation filmand the polarizing plate described later in such a manner that themaximum transmittance can be obtained when the liquid crystal moleculesare rotated from the initial orientation direction to about 45° in thedirection of the electric field, and transmitted light havingsubstantially no dependency on the wavelength within the range ofvisible light can be obtained.

The thickness of the liquid crystal composition layer (gap) iscontrolled with polymer beads having been subjected to a verticalorientation treatment, whereby the orientation of the liquid crystalmolecules in the vicinity of the beads upon displaying black isstabilized to obtain a good black level, so as to improve the contrastratio.

The specific resistance of the liquid crystal is from 1.0×10¹⁰ to1.0×10¹² Ω·cm (5.2×10¹¹ Ω·cm in this example). According to the system,the voltage charged between the pixel electrode and the counterelectrode can be sufficiently maintained even when the resistance of theliquid crystal is low.

The lower limit thereof is 1.0×10⁹ Ω·cm, and preferably 1.0×10¹⁰ Ω·cm.This is because the pixel electrode and the counter electrode areconstituted on the same substrate. When the resistance is too high,static charge formed during the production process is difficult to berelaxed, and therefor it is 1.0×10¹³ Ω·cm or less, and preferably1.0×10¹² Ω·cm or less.

Orientation Film

Polyimide is used as the orientation film ORI. The initial orientationdirections RDR of the upper and the lower substrates are parallel toeach other. As a method for applying the initial orientation direction,rubbing is generally employed, and oblique vapor deposition may also beused.

The relationship between the initial orientation direction RDR and theapplied electric field direction EDR is shown in FIG. 5. In thisexample, the initial orientation direction RDR is about 75° with respectto the horizontal direction. In the constitution of this example usingthe liquid crystal composition having negative dielectric anisotropy,the angle formed between the initial orientation direction RDR and theapplied electric field direction EDR is necessarily 45° or more and lessthan 90°. The orientation film is formed to have a thickness of from 20nm to 300 nm (about 100 nm in this example).

Polarizing Plate

A polarizing plate having electroconductivity is used as the polarizingplates POL1 and POL2. The polarized light transmitting axis MAX1 of theupper polarizing plate POLL agrees to the initial orientation directionRDR, and the polarized light transmitting axis MAX2 of the lowerpolarizing plate POL2 is perpendicular thereto. The relationships areshown in FIG. 5.

According to the configuration, normally close characteristics can beobtained in that the transmittance is increased associated with increaseof the voltage applied to the pixel of the invention (the voltagebetween the pixel electrode PX and the counter electrode CT). When novoltage is applied, black display of good quality can be obtained.

In this example, countermeasures for display failure caused by externalstatic charge and EMI are conducted by imparting electroconductivity tothe polarizing plates. The electroconductivity is preferably a sheetresistance of 10⁸ Ω per square or less when the countermeasure only forstatic charge is necessary, and is preferably a sheet resistance of 10⁴Ω per square or 1 ss when the countermeasure for EMI is also necessary.It is also possible to provide an electroconductive layer on the surfaceof the glass substrate opposite to the surface having the liquid crystalcomposition supported thereon (i.e., the surface having the polarizingplate is adhered).

Constitution Around Matrix

FIG. 6 is a plane view showing an important part around a matrix (AR) ofthe display panel PNL containing the upper and lower glass substratesSUB1 and SUB2. FIG. 7A is a cross sectional view showing the part aroundthe external connecting terminal GTM, to which a scanning circuit is tobe connected, and FIG. 7B is a cross sectional view showing the partaround the seal part having no external connecting terminal.

Upon production of the panel, plural devices are worked on one glasssubstrate, followed by dividing, to improve the throughput when the sizeis small, and when the size is large, in order for common use of theproduction equipments, a glass substrate of the standardized size isworked for any kind of product, followed by cutting into the size forthe respective kinds of product.

In any case, the glass is cut after subjecting the predeterminedprocess. FIGS. 6, 7A and 7B show the latter example and shows the stateafter cutting the upper and lower substrates SUB1 and SUB2. The symbolLN in FIG. 6 means the edges of the substrates before cutting.

In any case, the size of the upper substrate SUB2 is limited inside thelower substrate SUB1 in the finalized state in such a manner that thepart having the external connecting terminals Tg and Td and the terminalCTM (the upper periphery and the left periphery in FIG. 6) is exposed.

The terminals Tg and Td refer to a plurality of units of tape carrierpackages TCP (see FIG. 13) containing a scanning circuit connectingterminal GTM and a drain signal circuit connecting terminal DTM, both ofwhich will be described later, as well as outgoing line parts thereofmounted on an integrated circuit chip CHI (see FIG. 13).

The groups of the outgoing lines from the matrix part to the externalconnecting terminal part is inclined toward the both ends thereof. Thisis because the terminals DTM and GTM of the display panel PNL areconformed to the intervals of the arrangement of the package TCP and theconnecting terminals of the package TCP.

The counter electrode terminal CTM is a terminal for applying the commonvoltage from an external circuit to the counter electrode CT. The commonvoltage signal line CL in the matrix part is drawn to the opposite sideof the scanning circuit terminal GTM (the right side in FIG. 6), and therespective common voltage signal lines are integrated to a common basline CB, which is connected to the counter electrode terminal CTM.

A seal pattern SL is formed along the edges of the transparent glasssubstrates SUB1 and SUB2 to seal the liquid crystal CL except for aliquid crystal inlet INJ. The sealing material comprises, for example,an epoxy resin. The layers of the orientation films ORI1 and ORI2 areformed inside the seal pattern SL. The polarizing plates POL1 and POL2are formed on the outer surfaces of the lower transparent glasssubstrate SUB1 and the upper transparent glass substrate SUB2,respectively. The liquid crystal LC is filled in the region comparted bythe seal pattern SL between the lower orientation film ORI1 and theupper orientation film ORI2 setting the direction of the liquid crystalmolecules. The lower orientation film ORI1 is formed above theprotective film PSV on the side of the lower transparent glass substrateSUB1.

The liquid crystal display device is fabricated in such a manner thatthe various layers are accumulated separately on the side of the lowertransparent glass substrate SUB1 and the side of the upper transparentglass substrate USB2, and the lower transparent glass substrate SUB1 andthe upper transparent glass substrate SUB2 are superimposed each other.Thereafter, the liquid crystal LC is filled from the opening INJ of theseal material SL, and the injection opening INJ is sealed, for example,with an epoxy resin, followed by cutting the upper and lower substrates.

Gate Terminal Part

FIGS. 8A and 8B are diagrams showing the connecting structure from thegate signal line of the display matrix to the external connectionterminal GTM thereof. FIG. 8A is a plane view, and FIG. 8B is a crosssectional view on line B-B in FIG. 8A.

FIGS. 8A and 8B correspond to the left lower part of FIG. 6, and thepart of the inclined line is expressed by a straight line forconvenience. In FIGS. 8A and 8B, hatching is given to the Cr—Mo layer g1for easy understanding.

The gate terminal GTM comprises the Cr—Mo layer g1 and a transparentelectroconductive layer ITO1 for protecting the surface of the Cr—Molayer and for improving the reliability of the connection to the TCP(tape carrier package).

The transparent electroconductive layer ITO1 is formed with atransparent electroconductive film ITO. As shown in FIG. 8B, theinsulating film G1 and the protective film PSV are formed on the rightside of FIG. 8B, and the terminal part GTM on the left edge is exposedfrom the insulating film GI and the protective film PSV to enableelectric contact with an outer circuit.

While only one pair of the gate line GL and the gate terminal GTM isshown in FIGS. 8A and 8B, plurality of the pairs are actually arrangedin the vertical direction to constitute a group of the terminals Tg (seeFIG. 10), and the left side in FIGS. 8A and 8B of the gate terminal GTMis extended beyond the cutting region of the substrates during theproduction process and is shorted with a short circuit line SHg (notshown in the figure). The short circuit formed by the short circuit lineSHg prevents electrostatic damage of the orientation film ORI1 due torubbing during the production process.

Drain Terminal DTM

FIGS. 9A and 9B are diagrams showing the connecting structure from thedrain signal line DL to the external connecting terminal DTM thereof.FIG. 9A is a plane view, and FIG. 9B is a cross sectional view on lineB-B in FIG. 9A. FIGS. 9A and 9B correspond to the right upper part ofFIG. 6, and the right end corresponds to the upper end of the substrateSUB1 while the aspect of the figures is changed for convenience.

The external connecting drain terminal DTM is arranged in the verticaldirection, and the drain terminal DTM constitutes the group of terminalsTd (suffix omitted) as shown in FIG. 13 and are extended beyond thecutting line of the substrate SUB1. The drain terminal DTM is extendedbeyond the cutting region of the substrate during the productionprocess, and all of them are shorted with a short circuit line SHd (notshown in the figure) for preventing electrostatic damage.

The drain connecting terminal DTM is formed with a transparentelectroconductive layer ITO1 and is connected to the drain signal lineDL at the part where the protective film PSV is removed. The transparentelectroconductive film ITO1 is formed with a transparentelectroconductive film ITO as similar to the case of the gate terminalGTM. The outgoing line from the matrix part to the drain terminal partDTM is formed with the layer d1, which is the same level as the drainsignal line DL.

Counter Electrode Terminal CTM

FIGS. 10A and 10B are diagrams showing the connecting structure from thecommon voltage signal line CL to the external connecting terminal CTMthereof. FIG. 10A is a plane view, and FIG. 10B is a cross sectionalview on line B-B in FIG. 10A. FIGS. 10A and 10B correspond to the leftupper part of FIG. 6.

The respective common voltage signal lines CL are integrated to a commonbas line CB, which is withdrawn to the counter electrode terminal CTM.The common bas line CB has such a structure that an electroconductivelayer g3 (not shown in the figure) is accumulated on theelectroconductive layer g1, which are electrically connected with thetransparent electroconductive layer ITO1.

This is because the resistance of the common bas line CB is decreased,and the common voltage is sufficiently supplied from an external circuitto the respective common voltage signal lines CL. The structure has suchcharacteristics that the resistance of the common bas line can bedecreased without addition of another electroconductive layer.

The counter electrode terminal has such a structure that the transparentelectroconductive layer ITO1 is accumulated on the electroconductivelayer g1. The transparent electroconductive film ITO1 is formed with atransparent electroconductive film ITO as similar to the cases of theother terminals.

The electroconductive layer g1 is covered with the transparentelectroconductive layer ITO1 to protect the surface thereof and toprevent electric corrosion thereof.

The connection of the transparent electroconductive layer ITO1 with theelectroconductive layer g1 and the electroconductive layer d1 iseffected through a through hole formed via the protective film PSV andthe insulating film GI.

Total Equivalent Circuit of Display Device

A wiring diagram of the equivalent circuit of the display matrix partand a peripheral circuit thereof is shown in FIG. 11. While FIG. 11 is acircuit diagram, it is drawn to correspond to the actual geometricarrangement.

Plural pixels are two-dimensionally arranged to form a matrix array. InFIG. 11, the symbol X denotes the drain signal line DL, and the suffixesG, B and R are attached to correspond to a green pixel, a blue pixel anda red pixel, respectively. The symbol Y denotes a gate signal line GL,and the suffixes 1, 2, 3 to end are attached to correspond to the orderof scanning timing.

The gate signal line Y (suffix omitted) is connected to a verticalscanning circuit V, and the drain signal line X (suffix omitted) isconnected to a drain signal driving circuit H. The symbol SUP denotes acircuit containing a power source circuit for obtaining pluralstabilized power sources obtained by dividing one power source and acircuit for converting display information from a host (host operationprocessing device) for a CRT (cathode ray tube) to display informationfor a TFT liquid crystal display device.

Driving Method

FIG. 12 shows a driving waveform of the liquid crystal display device ofthis example. A gate signal VG takes an on-level per one scanningperiod, and the others take an off-level. A drain signal voltage VD isapplied in such a manner that a positive pole and a negative pole areapplied to one pixel by inverting per one flame at an amplitude of twicethe voltage to be applied to the liquid crystal layer.

The polarity of the drain signal voltage VD is inverted per one columnand is also inverted per two lines. Accordingly, pixels having invertedpolarities are arranged adjacent to each other in the vertical andhorizontal directions (i.e., dot inversion driving), and thus flickerand cross talk (smear) are difficult to be formed.

The common voltage Vc is set at a voltage below the center voltage ofthe polarity inversion of the drain signal voltage by a constant amount.This is to compensate the feedthrough voltage formed upon changing thethin film transistor TFT from the on-state to the off-state, and isconducted by applying an alternating current voltage VLC having lessdirect current component to the liquid crystal. (A liquid crystalsuffers severe after image and deterioration when a direct current isapplied thereto.)

Surface Panel PNL and Driving Circuit Board PCB1

FIG. 13 shows a top view showing the sate where the display panel PNLshown in FIG. 6 having the drain signal driving circuit H and thevertical scanning circuit V connected thereto.

The symbol CHI denotes a driving IC chips for driving the display panelPNL (in which the lower five chips are driving IC chips for the verticalscanning circuit, and the left ten chips are driving IC chips for thedrain signal driving circuit).

The symbol TCP denotes a tape carrier package having the driving ICchips CHI are mounted by a tape automated bonding method (TAB), and PCB1denotes a driving circuit board having the TCP and capacitors mountedthereon, which is divided into two, i.e., one for the drain signaldriving circuit and the other for the gate signal driving circuit.

The symbol FGP denotes a flame ground pad, which is soldered to a springform fragment provided by cutting a shield case SHD. The symbol FCdenotes a flat cable connecting the lower driving circuit board PCB1 andthe left driving circuit board PCB1.

As the flat cable FC, one comprising plural lead wires (comprisingphosphor bronze plated with Sn) supported by sandwiching with apolyethylene layer in a stripe form and a polyvinyl alcohol layer isused.

Production Process

The production process of the substrate SUB1 of the liquid crystaldisplay device described in the foregoing will be described withreference to FIGS. 14A to 14C and 15D to 15F below. In FIGS. 14A to 14Cand 15D to 15F, abbreviated names of the steps are shown in the centerof the figures, and the work flow is shown by cross sectional views, inwhich the thin film transistor TFT part shown in FIG. 3 is shown on theleft side, and the part around the gate terminal shown in FIGS. 8A and8B is shown on the left side.

The steps A to F are divided corresponding to the respectivephotographic processes, and the cross sectional views of the steps showthe stage where the treatment after the photographic process iscompleted, and the photoresist has been removed.

The photographic process referred herein means a series of operationsincluding coating of a photoresist, selective exposure through a maskand development thereof. The steps A to C will be described withreference to FIGS. 14A to 14C, and the steps D to F will be describedwith reference to FIGS. 15D to 15F, but repetition of explanation willbe omitted.

(a) Step A

An electroconductive film ITO1 comprising ITO having a film thickness of100 .ANG. is provided on a lower transparent glass substrate SUB1comprising AN635 Glass (a trade name) by sputtering. After subjecting toa photographic process, the electroconductive film ITO1 is selectivelyetched with an HBr solution, so as to form a counter electrode CT.

(b) Step B

An electroconductive film g1 comprising Cr having a film thickness of200 nm is provided by sputtering. After subjecting to a photographicprocess, the electroconductive film g1 is selectively etched with cericnitrate ammon, so as to form a gate electrode GT, a gate signal line GL,a common voltage signal line CL, a gate terminal GTM, a firstelectroconductive layer of a common bas line CB, a firstelectroconductive layer of a counter electrode terminal CTM1 and a basline SHg (not shown in the figure) connecting the gate terminal GTM. Thematerial of the electrode is not limited to Cr, and Mo, Ti, Ta, W and analloy thereof may be used.

(c) Step C

An ammonia gas, a silane gas and a nitrogen gas are introduced into aplasma CVD apparatus to provide a silicon nitride film having a filmthickness of 350 nm. A silane gas and a hydrogen gas are introduced intothe plasma CVD apparatus to provide an i type amorphous Si film having afilm thickness of 120 nm is provided, and then a hydrogen gas and aphosphine gas are introduced into the plasma CVD apparatus to provide anN(+) type amorphous Si film having a film thickness of 30 nm.

After subjecting to a photographic process, the N(+) type amorphous Sifilm and the i type amorphous Si film are selectively etched by usingSF₆ and CCl₄ as a dry etching gas, so as to form islands of an i typesemiconductor layer AS.

(d) Step D

An electroconductive film d1 comprising Cr having a film thickness of 30nm is provided by sputtering. After subjecting to a photographicprocess, the electroconductive film d1 is etched by the same liquid asin the step B, so as to form a drain signal line DL, a source electrodeSD1, a drain electrode SD2, a first electroconductive layer of a commonbas line CB2 and a bas line SHd (not shown in the figure) shorting thedrain terminal DTM. The material of the electrode is not limited to Cr,and Mo, Ti, Ta, W and an alloy thereof may be used.

The N(+) type amorphous Si film is etched by introducing CCl₄ and SF₆into a dry etching apparatus, so as to selectively remove the N(+) typesemiconductor layer d0 between the source and the drain.

After patterning the electroconductive film d1 with a mask pattern, theN(+) type semiconductor layer d0 is removed by using theelectroconductive film d1 as a mask. That is, the N(+) typesemiconductor layer d0 remaining on the i type semiconductor layer AS isremoved in a self aligning manner except for the part where theelectroconductive film d1 is present. At this time, since the wholethickness of the N(+) type semiconductor layer d0 is removed by etching,the surface part of the i type semiconductor layer AS is also slightlyetched, and the extent thereof can be controlled by the etching time.

(e) Step E

An ammonia gas, a silane gas and a nitrogen gas are introduced into theplasma CVD apparatus to provide a silicon nitride film having a filmthickness of 0.4 μ. After subjecting to a photographic process, thesilicon nitride film is selectively etched by using SF₆ as a dry etchinggas, so as to pattern a protective film PSV and an insulating film GI.

(f) Step F

An electroconductive film ITO2 comprising ITO having a film thickness of12 nm is provided by sputtering. After subjecting to a photographicprocess, the electroconductive film ITO2 is selectively etched with anHBr solution, so as to form a pixel electrode PX.

Dissociative Dopant

The characteristic feature of this example is that 100 ppm of2,5-dimethylphenol is added to the mother liquid crystal. The motherliquid crystal has a specific resistance of 1.9×10¹³ Ω·cm and an NIpoint of 70.5° C. When 2,5-dimethylphenol shown by the followingstructural formula is added thereto, the specific resistance becomes5.2×10¹¹ Ω·cm. The NI point of the liquid crystal is 70.4° C., which issubstantially the same as the liquid crystal before the addition. 3

2,5-Dimetnylphenol

The dissociative dopant used in this example means an acidicdissociative substance or a basic dissociative substance, or in otherwords, a substance generating an H⁺ ion through dissociation by itselfin a polar solvent or generating an OH⁻ ion through a reaction withwater.

Specific examples thereof include a carboxylic acid (including ananhydride thereof), an amide, an amine and an alcohol. When thesesubstances are added to a liquid crystal, the ion concentration in theliquid crystal is increased, so as to decrease the specific resistance.

Evaluation of display quality of the liquid crystal display device ofExample 1 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 2

Example 2 of a liquid crystal display device according to the inventionis the same as Example 1 except that the addition amount of the dopantused is changed to 1,000 ppm. The mother liquid crystal has a specificresistance of 1.9×10¹³ Ω·cm and an NI point of 70.5° C. When2,5-dimethylphenol is added thereto, the specific resistance becomes2.5×10¹⁰ Ω·cm. The NI point of the liquid crystal is 70.2° C., which issubstantially the same as the liquid crystal before the addition.

Evaluation of display quality of the liquid crystal display device ofExample 2 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

CL EXAMPLE 3

Example 3 of a liquid crystal display device according to the inventionis the same as Example 1 except that the thickness of the orientationfilm used is changed to 50 nm.

Evaluation of display quality of the liquid crystal display device ofExample 3 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 4

Example 4 of a liquid crystal display device according to the inventionis the same as Example 1 except that the thickness of the orientationfilm used is changed to 300 nm.

Evaluation of display quality of the liquid crystal display device ofExample 4 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 5

Example 5 of a liquid crystal display device according to the inventionis the same as Example 1 except that the distance between the pixelelectrodes is 2 μ.

Evaluation of display quality of the liquid crystal display device ofExample 5 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 6

Example 6 of a liquid crystal display device according to the inventionis the same as Example 1 except that a nematic liquid crystal containinga liquid crystal molecule having a monocyanocyclohexane structure in themolecule is used. The mother liquid crystal has a specific resistance of3.5×10¹² Ω·cm and an NI point of 71.5° C. When 2,5-dimethylphenol isadded thereto, the specific resistance becomes 2.5×10¹¹ Ω·cm. The NIpoint of the liquid crystal is 71.2° C., which is substantially the sameas the liquid crystal before the addition.

Evaluation of display quality of the liquid crystal display device ofExample 6 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 7

Example 7 of a liquid crystal display device according to the inventionis the same as Example 1 except that 500 ppm of 2,5-dimethylanilineshown by the following structural formula is used as the dopant. Themother liquid crystal has a specific resistance of 1.9×10¹³ Ω·cm and anNI point of 70.5° C.

When 2,5-dimethylaniline is added thereto, the specific resistancebecomes 1.2×10¹¹ Ω·cm. The NI point of the liquid crystal is 70.1° C.,which is substantially the same as the liquid crystal before theaddition. 4

2,5-Dimetnylaniline

Evaluation of display quality of the liquid crystal display device ofExample 7 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 8

Example 8 of a liquid crystal display device according to the inventionis the same as Example 1 except that 2,000 ppm of 2,5-dimethoxyphenolshown by the following structural formula is used as the dopant. Themother liquid crystal has a specific resistance of 1.9×10¹³ Ω·cm and anNI point of 70.5° C.

When 2,5-dimethoxyphenol is added thereto, the specific resistancebecomes 1.2×10¹⁰ Ω·cm. The NI point of the liquid crystal is 70.3° C.,which is substantially the same as the liquid crystal before theaddition. 5

2,5-Dimethoxyphenol

Evaluation of display quality of the liquid crystal display device ofExample 8 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 9

Example 9 of a liquid crystal display device according to the inventionis the same as Example 1 except that 300 ppm of2,5-diethoxy-4-morphorinoaniline dihydrochloride shown by the followingstructural formula is used as the dopant. The mother liquid crystal hasa specific resistance of 1.9×10¹³ Ω·cm and an NI point of 70.5° C. When2,5-diethoxy-4-morphorinoaniline dihydrochloride is added thereto, thespecific resistance becomes 2.5×10¹¹⁺Ω·cm. The NI point of the liquidcrystal is 70.2° C., which is substantially the same as the liquidcrystal before the addition. 6

2,5-Diethoxy-4-morphorinoaniline Dihydrochloride

Evaluation of display quality of the liquid crystal display device ofExample 9 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

EXAMPLE 10

Example 10 of a liquid crystal display device according to the inventionis the same as Example 1 except that 900 ppm of4-(2,5-diethoxy-4-nitrophenyl) morphrinone shown by the followingstructural formula is used as the dopant. The mother liquid crystal hasa specific resistance of 1.9×10¹³ Ω·cm and an NI point of 70.5° C. When2,5-diethoxy-4-morphorinoaniline dihydrochloride is added thereto, thespecific resistance becomes 8.9×10¹⁰ Ω·cm. The NI point of the liquidcrystal is 70.2° C., which is substantially the same as the liquidcrystal before the addition. 7

4-(2,5-Diethoxy-4-nitrophenyl)morphrinone

Evaluation of display quality of the liquid crystal display device ofExample 10 according to the invention is conducted and reveals that highquality display is confirmed, and substantially no formation of afterimage failure is observed.

COMPARATIVE EXAMPLE 1

Comparative Example 1 of a liquid crystal display device is the same asExample 1 except that2-cyano-3-fluoro-5-(4-n-propyl-trans-cyclohexyl)phenol shown by thefollowing structural formula is used as the dopant. The mother liquidcrystal has a specific resistance of 1.9×10¹³ Ω·cm and an NI point of70.5° C. When 1,000 ppm of2-cyano-3-fluoro-5-(4-n-propyl-trans-cyclohexyl)phen-ol is addedthereto, the specific resistance becomes 3.3×10¹¹ Ω·cm. The NI point ofthe liquid crystal is 70.4° C., which is substantially the same as theliquid crystal before the addition. 8

2-Cyano-3-fluoro-5-(4-n-propyl-trans-cyclohexyl)phenol

Evaluation of display quality of the liquid crystal display device ofComparative Example 1 is conducted and reveals that high quality displayis confirmed, but an after image failure is observed.

COMPARATIVE EXAMPLE 2

Comparative Example 2 of a liquid crystal display device is the same asExample 1 except that2-cyano-3-fluoro-5-(4-n-propyl-trans-bicycl-ohexyl)phenol shown by thefollowing structural formula is used as the dopant. The mother liquidcrystal has a specific resistance of 1.9×10¹³ Ω·cm and an NI point of70.5° C. When 100 ppm of2-cyano-3-fluoro-5-(4-n-propyl-trans-bicyclohexyl)phen-ol is addedthereto, the specific resistance becomes 5.5×10¹² Ω·cm. The NI point ofthe liquid crystal is 70.3° C., which is substantially the same as theliquid crystal before the addition. 9

2-Cyano-3-fluoro-5-(4-n-propyl-trans-bicyclohexyl)phenol

Evaluation of display quality of the liquid crystal display device ofComparative Example 2 is conducted and reveals that high quality displayis confirmed, but an after image failure is observed.

FIG. 16 is a plane view of another example of the invention. In thisexample, a pixel electrode PX and counter electrodes CT1 and CT2 are ina wave form (a zigzag form). According to the constitution, two regions(domains) having reorientation states that are different from each otherin direction are formed, and thus they compensates by each otherinversion of coloring and gradation in oblique directions, whereby awide viewing angle can be obtained.

That is, the respective electrodes have a zigzag form having pluralcrooked parts in the running direction thereof, and one side on thecrooked part has an angle 0 with respect to the vertical direction RDRPin FIG. 16, whereas the other side on the crooked part has an angle1800°-θ.

According to the configuration, the two regions (domains) havingreorientation states that are different from each other in direction areformed, and thus they compensates by each other inversion of coloringand gradation in oblique directions, whereby a wide viewing angle can beobtained.

The vertical direction RDRP also shows the initial orientation directionof an orientation film ORI for a liquid crystal molecule having positivedielectric anisotropy (Np liquid crystal), and the horizontal directionRDRN shows the initial orientation direction of an orientation film ORIfor a liquid crystal molecule having negative dielectric anisotropy (Nnliquid crystal). In the pixel structure of this example, both the liquidcrystal molecules having positive dielectric anisotropy and negativedielectric anisotropy.

In FIG. 16, a gate insulating film GI is formed between the pixelelectrode PX and the counter electrodes CT1 and CT2 as similar to theother examples, and an electric field in the horizontal directionrotating the liquid crystal molecule is formed between the electrodes.

A gate signal line GL and a drain signal line DL are the same as in theother examples. An amorphous semiconductor layer ASI is arranged betweenan electrode SD2 formed from the drain signal line DL as overlapping thegate signal line and an electrode SDl connected to the pixel electrodefor applying a storage voltage, so as to function as a thin filmtransistor TFT.

A protective film PSV is formed on the thin film transistor TFT. Theprotective film PSV is provided mainly for protecting the thin filmtransistor from moisture, and a film having high transparency and highmoisture resistance is used.

The protective film PSV is formed, for example, with a silicon oxidefilm or a silicon nitride film formed by using a plasma CVD apparatus,or in alternative, with an acrylic resin or polyimide, to have a filmthickness of about from 0.1 μ to 3 μ.

A counter voltage signal line CL is formed by the same production stepas the gate electrode, the scanning signal line GL and the counterelectrode CT, and is constituted as capable of electrically connectingwith the counter electrode CT. A counter voltage Vcom is supplied froman external circuit to the counter electrode CT via the counter voltagesignal line CL.

The part crossing the image signal line DL is narrowed to decrease theprobability of shorting with the image signal line DL, and may be formedin a bifurcated form, whereby they can be separated by laser trimmingeven when they form a short circuit.

An electrode ST is formed with a metallic film (a layer containingmetallic atoms) and is connected to the pixel electrode PX via a throughhole TH1. Furthermore, it is necessary that a potential is supplied tothe electrode ST from the outside, and a floating electrode exhibits noeffect. Therefore, it is connected to the other electrode by opening thethrough hole TH1 in the protective film PSV.

In this example, the electrode ST formed as integrated with the pixelelectrode PX overlaps the counter electrode CT2 via the protective filmPSV.

In order to ensure the contact even when scattering occurs on productionof the through hole and the electrode ST, the pixel electrode PX isprovided on a base larger than the pixel electrode that is provided at apart meeting the through hole TH1 at the end of the pixel electrode tobe integrated with the pixel electrode PX.

As described in the foregoing, in this example, the electrode STelectrically connected to the pixel electrode is formed on theprotective film PSV. According to the configuration, a capacitance (aprotective film capacitance), which is formed consequently between thepixel electrode PX and the counter electrodes CT1 and CT2 with theprotective film PSV or the protective film PSV and the insulating filmGI as a dielectric material, is charged through the electrode ST, andwhen an electrode having the same direct current potential (in the caseof alternating current, the potential of the direct current component)as the electrode ST is exposed to the liquid crystal layer due to aforeign matter, no charging current flows.

Therefore, no electrochemical reaction (electrode reaction) occurs inthe vicinity of the exposed electrode. That is, since the electrode STis formed on the protective film PSV, the charging current to theprotective film capacitance of the other electrode due to a defect ofthe protective film on the electrode is suppressed, so as to suppressformation of spot type brightness difference area.

Particularly, in the invention, the gate electrode GT or the scanningsignal line is defined as an electrode or a line on the cathode side.Furthermore, an electrode or a line having a higher potential than thegate electrode GT or the scanning signal line GL are defined as anelectrode or a line on the anode side, and the electrode or the line onthe anode side includes the source electrode SD1, the drain electrodeSD2, the image signal line DL, the pixel electrode PX, the counterelectrodes CT1 and CT2, and the counter voltage signal line CL.

As described in the foregoing, in the invention, while the electrode STis electrically connected to the pixel electrode as an example of theelectrode or the line on the anode side, the electrode ST may beconnected to an electrode or a line comprising one or both the cathodeand the anode.

FIGS. 17A, 17B, 18A, 18B, 19A and l9B are plane views showing modifiedexamples of the pixel described in FIG. 16. In FIGS. 17A, 17B, 18A, 18B,19A and 19B, the pixel electrode PX and the counter electrode CT each isformed with a transparent electrode (ITO or IZO), and the number of theelectrodes, the electrode interval and the layer, in which the electrodeis formed, are changed.

In FIGS. 17A, 17B, 18A, 18B, 19A and 19B, the electrodes of therespective groups of electrodes have a zigzag form having plural crookedparts in the running direction thereof. One side on the crooked part hasan angle θ with respect to the direction of the image signal line DL,whereas the other side of the crooked part has an angle 180°-θ.

According to the configuration, the two regions (domains) havingreorientation states that are different from each other in direction areformed, and thus they compensates by each other inversion of coloringand gradation in oblique directions, whereby a wide viewing angle can beobtained.

In FIGS. 17A and 17B, it is constituted in such a manner that thedirection of the electric field between the pixel electrode and thecounter electrode is directed to the direction crossing the image signalline DL. FIG. 17A shows the case where the pixel electrode PX comprisessix pieces, and the counter electrode CT is formed in a solid form onthe whole pixel region.

In FIG. 17A, the electrode ST is connected to the counter electrode andis extended outward to the boundary of the black matrix BM (shown by theouter dotted line in FIG. 17A) and inward to the inner dotted line.

FIG. 17B shows the case where the pixel electrode PX comprises sixpieces, and the counter electrode CT overlaps alternately the pixelelectrode and has a width that is larger than the pixel electrode. Incomparison to the case of FIG. 17A, the capacitance of the storagecapacitance can be made as small as 400 fF.

In FIGS. 18A, 18B, 19A and 19B, it is constituted in such a manner thatthe direction of the electric field between the pixel electrode and thecounter electrode is directed to the direction along the image signalline DL. In FIGS. 18A and 18B, the counter electrode CT is formed on thesubstantially whole pixel region. The electrode width, the number of theelectrodes and the electrode interval are changed between FIG. 18A andFIG. 18B.

Specifically, the width of the pixel electrode PX is 5 μ in FIG. 18A,whereas the width of the pixel electrode in FIG. 18B is 9 μ. The numberof the pixel electrode in one pixel is 30 in FIG. 18A.

The electrode interval is 5 μ in FIG. 18A, whereas it is 4 μ in FIG.18B. In FIG. 18B, the transmittance is improved in comparison to FIG.18A. FIGS. 18A, 18B, 19A and 19B contain an s-shape where the end of thepixel electrode is inverted up and down inside one pixel. This isbecause the auxiliary capacitance between the pixel electrode and thecounter electrode is made uniform in the direction of the scanningsignal line GL.

FIG. 19A shows the case where the pixel electrode PX has a width of thepixel electrode of 5 μ and an interval to the adjacent pixel electrodePX of 5 μ, and the counter electrode CT overlaps alternately the pixelelectrode and has a width that is larger than the pixel electrode PX.

In FIG. 19B, the pixel electrode PX has a width of 4 μ and an intervalto the adjacent pixel electrode PX of 4 μ. The opening ratio in FIG. 19Bis improved in comparison to FIG. 19A. The constitution of the pixel inFIGS. 19A and 19B can reduce the auxiliary capacitance in comparison tothe constitution of the pixel in FIGS. 18A and 18B.

It is understood from the foregoing examples that an active matrixliquid crystal display device causing less after image can be provided.

As described in the foregoing, according to the invention, such anactive matrix liquid crystal display device can be provided in that whena liquid crystal driving voltage wave having a direct current voltageoverlaid is applied to a liquid crystal layer, a direct current voltageremaining in the liquid crystal layer after removing the applied directcurrent voltage is removed to suppress an after image.

1. An active matrix type liquid crystal display device comprising: apair of substrates with liquid crystal layer there between; a pluralityof first signal lines and a plurality of second signal lines whichcrossing each other formed on one of the pair of substrates; asemiconductor layer formed above the first signal lines and under thesecond signal lines; a pixel electrode formed in each pixel region onthe one of the pair of substrates and electrically connected to one ofthe second signal lines; wherein the semiconductor layer has paralleledges and slant edges with respect to said one of the second signallines, and edges facing toward the semiconductor layer of the pixelelectrode in a crossing region where said one of the second signal linescrossing with one of the first signal lines are shaped substantiallyconforming with neighboring edges of the semiconductor layer to minimizechange of distance between the semiconductor layer and the pixelelectrode.
 2. An active matrix type liquid crystal display deviceaccording to claim 1, wherein the semiconductor layer has two slantportions defined between two pairs of said slant edges to be narrowergradually when extending away from the crossing region where said one ofthe second signal lines crossing with said one of the first signallines.
 3. An active matrix type liquid crystal display device accordingto claim 2, wherein the edges of the pixel electrode next to thesemiconductor layer is parallel to the neighboring edges of thesemiconductor layer.
 4. An active matrix type liquid crystal displaydevice according to claim 3, wherein said one of the first signal linesis narrower in the crossing region with said one of the second signallines than in a region overlapping with the pixel electrode.
 5. Anactive matrix type liquid crystal display device according to claim 2,wherein said one of the first signal lines is narrower in the crossingregion with said one of the second signal lines than in a regionoverlapping with the pixel electrode.
 6. An active matrix type liquidcrystal display device according to claim 4, further comprising acounter electrode connected to said one of the first signal lines,wherein the pixel electrode overlaps the counter electrode with aninsulating layer sandwiched therebetween.
 7. An active matrix typeliquid crystal display device according to claim 6, wherein the pixelelectrode has at least two forming regions, which define a non formingregion therebetween, between every two neighboring second signal linesof the second signal lines in the direction perpendicular to the secondsignal lines.
 8. An active matrix type liquid crystal display deviceaccording to claim 7, wherein the counter electrode is formed in boththe forming regions and the non forming region of the pixel electrode.9. An active matrix type liquid crystal display device according toclaim 8, wherein the counter electrode and the pixel electrode are madeof a transparent conducting material.
 10. An active matrix type liquidcrystal display device according to claim 5, further comprising acounter electrode connected to said one of the first signal lines,wherein the pixel electrode overlaps the counter electrode with aninsulating layer sandwiched therebetween.
 11. An active matrix typeliquid crystal display device according to claim 10, wherein the pixelelectrode has at least two forming regions, which define a non formingregion therebetween, between every two neighboring second signal linesof the second signal lines in the direction perpendicular to the secondsignal lines.
 12. An active matrix type liquid crystal display deviceaccording to claim 11, wherein the counter electrode is formed in boththe forming regions and the non forming region of the pixel electrode.13. An active matrix type liquid crystal display device according toclaim 12, wherein the counter electrode and the pixel electrode are madeof a transparent conducting material.